1. Field of the Invention
The present invention relates to a light emitting diode and the manufacturing method thereof. More particularly, the invention is directed to a light emitting diode having an adhesive layer and a reflective layer and the manufacturing method thereof.
2. Description of the Prior Art
Light emitting diodes can be used in a wide variety of devices, for example, optical displays, traffic lights, data storage devices, communication devices, illumination devices, and medical devices. To manufacture a light emitting diode of higher brightness is an important task of engineers.
A prior art method for improving LED brightness involves bonding two semiconductor parts together by van der Waals forces. However, it has a disadvantage in that van der Waals forces are too weak to provide a sufficient mechanical bonding strength between the two parts and therefore they are apt to separate.
In U.S. Pat. No. 5,376,580, a method for bonding an LED stack and a transparent substrate to create an ohmic interface therebetween is disclosed. The transparent substrate can be made of GaP. The light generated from the LED stack can pass through the LED stack as well as the transparent substrate. However, this prior art method has to be carried out at about 1000° C. by exerting a coaxial compressive force on the LED stack and the transparent substrate to form an ohmic interface therebetween. The primary disadvantage of this prior art method lies in that the property of the LED is destroyed by the high temperature during the manufacturing process and this results in an LED of low light emitting efficiency. In addition, the transparent GaP substrate has a color and a transparency of only about 60–70%. It therefore reduces brightness of the LED.
Another prior art method for improving LED brightness involves a bonding technique using a metal layer to bond an LED stack and a substrate. The metal layer forms a bonding layer and a mirror through its metallic property. Thereby, the light rays emitted from the LED stack can be reflected at the metal layer and re-enter the LED stack without passing through the metal layer and entering the substrate. The disadvantage that the some light rays are absorbed by a substrate can therefore be avoided. In such a manufacturing process, the bonding temperature of the metal layer is only about 300–450° C. The LED property will not be destroyed at these low temperatures. However, this bonding technique involves a few disadvantages. One of the disadvantages lies in that although a low bonding temperature will not cause any reaction between the metal layer and any of the two semiconductor layers to be bonded and therefore a highly reflective metal surface (reflectivity over 90%) and improved light emitting efficiency can be obtained, the bonding effect is not sufficient due to that there is no reaction between the metal layer and any of the semiconductor layers to be bonded, and an ohmic interface cannot be formed between the metal layer and any of the semiconductor layers to be bonded. Nevertheless, in case that a higher bonding temperature is adopted, the bonding between the metal layer and any of the two semiconductor layers to be bonded is good. However, the reflectivity of the reflective metal layer will be greatly reduced and therefore the metal layer cannot provide a good mirror function. This is another disadvantage of the bonding technique.
To avoid the aforementioned disadvantages, the inventors of the present application got an inventive concept to be explained in the following. In case a transparent adhesive layer is used for adhering a metal layer, as mentioned above, to an LED stack, light rays generated by the LED stack may pass through the transparent adhesive layer, be reflected by the metal layer, and then pass through the LED stack. However, if the metal layer is simply adhered to the LED stack by use of an adhesive layer, the adhesion between them is achieved only by van der Waals forces and peeling is apt to occur at the adhesion interface. The inventive concept lies in that a reaction layer is formed between the transparent adhesive layer and any of the LED stack and the metal layer, wherein a reaction occurs between the reaction layer and the transparent adhesive layer so that hydrogen bonds or ionic bonds are formed to enhance the bonding forces provided by the transparent adhesive layer. Thereby, the transparent adhesive layer can provide an enhanced mechanical strength and thus the above-mentioned disadvantage of peeling can be avoided. In addition, using the transparent adhesive layer can avoid the above-mentioned disadvantage caused by the bonding between the metal layer and the LED stack. Moreover, a transparent conductive layer can be formed between the transparent adhesive layer and the LED stack for improving the efficiency of current spreading and thereby can enhance the brightness of the LED.